Sound reproducing apparatus



Aug. 6, 1957 J. v. CORNEY 2,802,054

' SOUND REPRODUCING APPARATUS Filed July 30, 1953 cla'oss- SOURCE A OVER A NETWORK /N VENTOE @AMZZM ATTORNEY United States Patent G SOUND REPRQDUCIN G APPARATUS John Victor Corney, London, England, assignor to Ferguson Radio Corporation Limited, London, England The present invention relates to sound reproducing apparatus of the kind comprising a cone-type moving-coil loudspeaker.

The invention will be described with reference to the accompanying drawing in which Fig. 1 is an explanatory circuit diagram,

Fig. 2 is a curve illustrating the behaviour of the circuit of Fig. 1,

Fig. 3 is a circuit diagram exemplifying the application of the invention to the circuit of Fig. 1, and

Fig. 4 is a block schematic circuit diagram of one embodiment of the invention.

It can be shown that the circuit diagram of Fig. 1 is approximately the electrical equivalent of a cone-type moving coil loudspeaker. In Fig. 1 the input terminals to the speech coil of the loudspeaker are represented by terminals 10 and 11, the resistance of the speech coil by a resistor 12 and the inductance of the speech coil by an inductor 13. The mass of the vibrating system of the loudspeaker is represented by an inductor 14 coupled to the inductor 13 by mutual inductance, the compliance of the vibrating system is representedby a capacitor 15, the frictional and eddy-current lossesbya resistor 16 and the radiation resistance by a resistor 17.

It canalso be shown that the variations of the impedance of the loudspeaker with change in frequency and measured at the terminals and 11 are of the character indicated by a curve 18 in Fig. 2 in which the ordinate represents the impedance .of the loudspeaker and the abscissa represents frequency.

It will be seen in Fig. 2 that the impedance rises sharply to a maximum at the frequency fiQ. This frequency is the mechanical resonance frequency of the vibrating system and is made as low as possible. It will also be seen that the impedance rises progressively above the frequency f2. This is due to the fact that the impedance of the loudspeaker is inductive at those frequencies and hence increases with increase in frequency.

By making f1 sufiiciently low the sharp increase in the impedance at that frequency can be made of no consequence. The increase in impedance occurring above the frequency f2 gives rise, however, to some difiiculties in the design of amplifiers for driving the loudspeaker.

For example sometimes two or more loudspeakers are connected to an amplifier, one of the loudspeakers being especially designed for reproducing relatively low audio frequencies and one or more of the remaining loudspeakers being especially designed for reproducing relatively high audio frequencies. With such a combination of loudspeakers a network known as a cross-over network is inserted between the amplifier and the loudspeakers, the cross-over network serving to separate the low frequency components from the high frequency components in the audio frequency voltages to be fed to the loudspeakers. The low frequency components are fed to the loudspeaker designed for reproducing low frequencies and thehigh frequency components are fed to the loudspeaker designed for reproducing high frequencies.

A common form of cross-over network is that known as the constant resistance type, examples of which are shown in Figs. 116 and 117 and described on page 251 of Radio Engineers Handbook, by F. E. Termampub'lished by the McGraw-Hill Book Company, Inc., 1943. A crossover network of this type is one whose input impedance is constant and resistive throughout a given frequency band (the audio frequency band or a given part thereof insofar as the present invention is concerned) provided the terminations of the network are resistive, of suitable values, and are equal and constant throughout the given band.

It will be seen, however, from Fig. 2 that these conditions are not met by a cone-type moving-coil loudspeaker. When using a cone-type moving-coil loudspeaker with a cross-over networkof the constant resistance type it may be found that the increase in impedance of the loudspeaker with increase in frequency degrades the performance of the network in the regional? the cross-over frequency and gives rise to objectionable aural effects.

In another example when a considerable amount of negative feedback is used in anamplifier the amplifier may be stable when provided with a resistive load but may tend to oscillate at high frequencies when used with a reactive load, as is provided by a loudspeaker of the moving-coil cone-type when connected to an amplifier either director by means ofa cross-over network and hence to produce objectionable aural effects.

I The objectof the present invention is to provide an impedance-correcting network for-loudspeakers of the moving-coil cone-type whereby the total impedance is made resistive and substantially constant over a substantial band of audio frequencies above the mechanical resonance frequency of the vibrating system of the loudspeaker, whereby the aforesaid objectionable aural effects can be overcome or substantially overcome.

According to the present invention a cone type movingcoil loudspeaker has a resistor and a capacitorconnected inseries between its input terminals, the resistor having a resistance R substantially equal to the effective resistance of the speech coil of the loudspeaker and the capacitor having a capacitance -C such that the equation 1/(21rfoC) :12 is substantially met, where ft) is the frequency above the mechanical resonance frequency represented at f1 at which the resistance R and the reactance 21rf0L of the speech coil of the loudspeaker are equal.

Thus ignoring the components 14,15, 16 and 17 of Fig. 1, for frequencies above in Fig. 2, the electrical equivalent of the loudspeaker according to the invention is as shown in Fig. 3, where 19 and 20 represent the resistor. and capacitor respectivelyconnected in series be tween the two input terminals 10 and 11.

Assuming R to be the resistance of the resistor 12 and L to be the inductance of the inductor 13 in Fig. 3 and f0 to be the frequency at which R=21rfoL, then at frequencies greater than fa Zs=R+j27rfL (i) where Z5 is the impedance of the resistor 12 and the inductor 13 in series and f is a frequency greater than is, and j=\/1.

Equation i may be re-written as Zs=R+(jf/f0)2fl'f0L (ii) Since R=21rf0L Equation ii may be re-written +J'f f By putting f/fo=x Equation iii may be re-written Zs=R(1+1'x) It can likewise be shown that Zc=R(1+1/jx) (v) It can be shown that this expression equals R.

For frequencies higher than fa, therefore, the combination shown in Fig. 3 has a constant resistance and zero phase angle.

Thus by connecting a resistor and capacitor of appropriate values in series between the terminals of a movingcoil cone-type loudspeaker the impedance presented by the loudspeaker to the output of an amplifier or cross-over network driving the loudspeaker is made constant and resistive over the whole of the audio frequency-band except at and near the mechanical resonance frequency of the vibrating system of the loudspeaker. Where two loudspeakers designed for reproducing relatively low and relatively high audio frequencies are each arranged in accordance with the invention the mechanical resonance frequency of the high frequency loudspeaker can be arranged to fall well below the cross-over frequency and the mechanical resonance frequency of the low frequency loudspeaker can be arranged to be a value sufficiently low to render the aural effects of the mechanical resonance of negligible consequence. Then by virtue of the compensating effect of an impedance-correcting network according to the invention a constant resistive impedance is presented by each compensated loudspeaker to the output terminals of the cross-over network in the critical range of frequencies at and near the cross-over frequency, and improved operation of the cross-over network and amplifier results.

Further according to the invention therefore, sound reproducing apparatus comprises a source of electrical oscillations to be reproduced connected through a constant resistance cross-over network to two cone-type moving-coil loudspeakers adapted for reproducing relatively high and relatively low audio frequencies respectively, and between the output of the cross-over network and the two loudspeakers are connected two impedance-correcting networks respectively, each comprising a resistor and capacitor connected in series across the input to the loudspeaker associated therewith, the resistor having a resistance R substantially equal to theeifective resistance 50 of the speech coil of the associated loudspeaker and the capacitor having a capacitance C such that the equation 1/ (21rf0C)=R is substantially met, where in is the frequency at which the resistance R and the reactance 21rfoL of the speech coil of the associated loudspeaker are equal.

This is illustrated in Figure 4. A source 17 of acoustic frequency oscillations to be reproduced, for instance a 21rfoL are then as measured in the radio receiver, is coupled through an amplifier 18 to the input of a crossover network 19. One output of the network 19 is connected to a cone-type moving-coil loudspeaker 20 adapted to reproduce high frequencies and the other output of the network 19 is connected to a cone-type moving-coil loudspeaker 21 adapted to reproduce lower frequencies. Across the inputs to the loudspeakers 20 and 21 are connected compensating networks 19', 20' and 19", 2 respectively, designed in the manner set forth.

It will be understood that the compensating network may be connected across the primary winding of a transformer whose secondary winding is connected to the speech coil of the loudspeaker, instead of direct across the speech coil terminals. The effective values of R and primary circuit of the transformer.

I claim:

1. A cone-type moving coil loudspeaker having a speech coil, terminals for the application of oscillations to be reproduced to said speech coil, and a resistor and a capacitor connected in series with one another between said terminals, said resistor having a resistance R substantially equal to the effective resistance of said speech coil and said capacitor having a capacitance C such that the equation l/(21rfnC) =R is substantially met, where in is the frequency above the mechanical resonance frequency of the loudspeaker system at which the resistance R and the reactance 21rfoL of said speech coil are equal.

2. Sound reproducing apparatus comprising two conetype moving-coil loudspeakers each having a pair of input terminals for electrical oscillations to be reproduced, and connected to the speech coil of the loudspeaker, the loudspeakers reproducing respectively relatively high and relatively low audio frequencies, a constant resistance crossover network, means connecting the two pairs of output terminals of said cross-over network to said input terminals respectively, and connected across each said pair of input terminals a resistor in series with a capacitor, each said resistor having a resistance R substantially equal to the effective resistance of the speech coil of the associated loudspeaker and each said capacitor having a capacitance C such that the equation l/(21rf0C) :R is substantially .met where ft] is the frequency above the resonance frequency of the respective cone-type moving-coil loudspeaker at which the resistance R and the reactance Z'rrfOL of the speech coil of the loudspeaker are equal.

References Cited in the file of this patent UNITED STATES PATENTS 1,772,374 Weinberger Aug. 5, 1930 2,007,748 Olson July 9, 1935 2,612,558 Klipsch Sept. 30, 1952 FOREIGN PATENTS 416.982 Great Britain Sept. 24, 1934 

